As a critical circuit unit in an analogy system and a mixing signal system, the operational amplifier is widely adopted in an analogy/digital converter (ADC), a souring filter, a waveform generator and a video frequency amplifier etc. In accomplice with the development of the CMOS technology, the dimension for the voltage source and the transistor channel is continuously miniaturized, so that the design for the operational amplifier becomes more and more complicated.
In an ADC, the operational amplifier is the most critical component. For example, certain un-ideal characteristics such as, finite gain, finite bandwidth and finite slew rate for the operational amplifier will cause incomplete charges shift in the integrator in the ADC and results in the nonlinearity effect for the ADC.
Thus there is a differential amplifier presented. The differential amplifier is an electronic amplifier which amplifies the voltage difference from two input ends by a stationary gain. The differential amplifier can not only effectively amplify direct current signal but can also effectively reduce zero drifting resulted from the power fluctuation and the temperature variation, and therefore it is in widespread use nowadays. In particular, the differential amplifier is massively adopted in the integrated operational circuit. The differential amplifier is often used as a pre-stage amplifier in a multi-stage amplifier, as an input level in a power amplifier and an emitter coupled logic (ECL) circuit. The differential amplifier possesses several advantages as follows: (1) It has high input resistance and low output resistance; (2) the output signal is in proportion to the difference of two input signals; and (3) the frequency respond is much broader than that in normal amplifier. A typical differential amplifier is a special direct coupled amplifier circuit, which requires an asymmetric layout which the electronic elements therein is arranged in asymmetric.
As compared with the single-ended operational amplifier, the conventional fully differential amplifier 10 usually used for transforming a single end signal into a differential signal as shown in FIG. 1 including a regular operational amplifier 11 further has many advantages such as the fully differential amplifier can provide larger output voltage swinging, is not easy to be influenced by noises resulted from the common mode, has better linearity, reduces even harmonics interferences and can simply the implementation of a basing circuit. However, although the fully differential amplifier 10 as shown in FIG. 1 has the aforementioned advantages, it is not widely applied, due to the lower bandwidth characteristic the fully differential amplifier has which requires to be incorporated with a common mode feedback (CMFB) circuit 12.
A conventional transistor-based amplifier as shown in FIG. 2 is a simple alternative scheme that uses transistors in replace of the operational amplifier to build a differential amplifier. The transistor-based differential amplifier is not such sensitive to be affected by the voltage difference between gate and source of the transistor and has a gain ratio
      G    ≅                  R        22                              R          21                +                  1          Gm                      ,where the Gm is a transconductance for an input end PMOS and has significant influence on gain ratio. Since the transconductance Gm is not a constant and varied in accordance with the manufacturing process, if it is desired to eliminate or minimize the influence of transconductance Gm upon the gain ratio, the resistance R21 should be much larger than
  1  Gmto render
      R    21    >>            1      Gm        .  If the transconductance Gm fails to be significantly raised, it must choose a resistor having a larger resistance R21. However, the resistor with larger resistance R21 will limit the amplifying performance for a transistor-based differential amplifier.
As could be known from FIG. 2, if the resistance R21 is increased by a multiple K over resistance R22, it is known that the influence of transconductance Gm upon the gain ratio will correspondingly become minor. In FIG. 2, the multiple K value is set as 2. However, in order to render the output signals Vin and Vip maintained on the same operating point and within a specific dynamic operating range, the electrical current flowing through the resistor 2*R21 must be reduced to
  1  Ktimes, and the total electrical current decrease quantity
      (          1      -              1        K              )    ⁢  Imay be added to Vin and Vip or compensated by Vin and Vip. FIG. 3 is a diagram illustrating a conventional transistor-based differential amplifier whose multiple K value is set as 4.
Therefore, it is tried to rectify those drawbacks and provide a better differential amplifier. The present invention provides a differential amplifier in order to achieve the foresaid objective.